1. Field of the Invention
This invention relates to hearing aids. More specific, it relates to hearing aids with more than one acoustic output transducer. The invention also relates to a processor for a hearing aid.
Hearing aids essentially comprise a microphone for picking up acoustic sound waves and converting them into electrical signals, electronic circuitry for amplifying the electrical signals generated by the microphone, and an acoustic output transducer for reproducing the amplified electrical signals. The amplifier may favor certain frequency bands in the audio spectrum to other frequency bands according to a prescription in order to compensate for an individual hearing loss.
In this application, the term “high frequencies” preferably refers to audio frequencies between 3 kHz and 15 kHz, and the term “low frequencies” preferably refers to audio frequencies between 20 Hz and 3 kHz.
2. The Prior Art
Hearing aids may be used to alleviate very different hearing impairments. Some examples of a hearing impairment are loss of a narrow band of frequencies, loss of the high frequencies, loss of low frequencies, or a more evenly distributed hearing loss across the entire audio spectrum. In cases where some residual hearing is present in the affected frequency range a hearing aid user may benefit from a hearing aid with means to process these frequencies.
Present-day hearing aids have a limited high-frequency reproduction, usually capped at about 4-8 kHz, mainly due to limitations of the output transducer. For reasons in the mechanical interactions in the components, extension of the frequency range only comes against the cost of a reduced output power in the low frequency end, and a trade off needs to be found somewhere. Transducers for use in hearing aids are manufactured with focus on speech reproduction, and thus optimized for use in the 200 Hz-6 kHz frequency range, important for speech recognition. However, other sounds of interest, e.g. sounds originating from animals or machinery, are present in the 6 kHz-15 kHz range, too. Individuals with normal hearing are usually able to perceive sounds up to between 15 kHz and 20 kHz, and even persons with a profound hearing loss may still possess some ability to perceive sounds above and beyond 8 kHz, dependent on the individual nature of the hearing loss.
Recent studies have shown that hearing-impaired young children still having residual hearing left in the 6 kHz-15 kHz range may benefit from the availability of this frequency range when learning to speak. In speech, the main part of the fricative sonic energy of the so-called morphemes /s/ and /z/, i.e. the speech sounds “s” and “z”, generally lies above 4 kHz, especially in the range of 4 kHz-8 kHz, and the ability to perceive and subsequently reproduce those sounds may be improved significantly if this frequency range is made available to hearing-impaired children under the circumstances mentioned earlier. A hearing aid having means to reproduce the frequency range from 200 Hz up to perhaps between 15 kHz and 20 kHz is thus desirable.
Dual acoustic transducers embodied as composite units are known. For instance, the EJ transducer series from Knowles Electronics, Inc. are dual magnetic receiver types configured for use in hearing aid applications. Such receivers comprise two essentially identical transducer units sandwiched together to form a single unit for use in a hearing aid. During manufacture, great care is taken in order to ensure that the two transducer units eventually perform as identically as possible with respect to their electrical and mechanical characteristics. Dual acoustic transducers are mainly used in applications where high sound pressure levels are required, for instance in high-power hearing aid applications.
U.S. Pat. No. 4,548,082 describes a hearing aid having two independently driven acoustic output transducers, denoted a woofer and a tweeter, respectively, for reproducing low-frequency and high frequency bands in the audible spectrum. The two acoustic output transducers are driven by a pair of sample-and-hold circuits, alternatingly sampling the output from a D/A converter for providing the acoustic output transducers with low-frequency and high-frequency sounds, respectively. The sample-and-hold circuits are controlled by a multiplexer providing the alternating signal feeds to the two acoustic output transducers. Optional anti-aliasing filters may be provided between the sample-and-hold circuits and the acoustic output transducers in order to filter out aliasing noises.
Although this approach provides means for driving more than one output transducer in a hearing aid, it also has some serious shortcomings. Driving an acoustic output transducer through a sample-and-hold circuit is very likely to introduce noise, and various spurious and aliasing effects, degrading the quality of the output and needing compensation.